Many different technical solutions exist in the food processing industry for grading delicate objects such as fish and shrimp. For such objects, all mechanical treatment needs to be gentle but at the same time demands for high-throughput laboring processing, including sorting according to size, are ever increasing.
Currently available grading apparatus include ridge-belt machines, such as have been developed earlier by the present inventors and are disclosed in WO 96/41541 and WO 98/48951, the whole contents of which are included herein by reference. Other similar apparatus are described in U.S. Pat. No. 4,723,660 and Norwegian Patent No. 132917. Ridge belt machines comprise juxtaposed conveyor belts arranged such that the width of channels between adjacent belts is wider at the unloading end of the belts than at the loading end. The items to be sorted are loaded at the loading end of the conveyors and transported by the conveyors towards the unloading end. Means for loading evenly and suitably the Items on the loading end have been developed and are described, e.g. in WO 98/48951. The items will fall through the channels inbetween the belts when the width of the item is approximately the same as the width of the channel. Thus, smaller items will fall through the channels earlier while larger items are transported further along the conveyors before they fall through the channels. By using such machines, items such as capelin or sardine have been sorted with 5 m long conveyors into three to four different size grades but the accuracy of the grading is limited.
Conventional ridge belt sorting machines have some drawbacks though and have a limited grading resolution. Optimally, items such as whole fish or shrimp are sorted according to their nominal width measured across the body of the item lying on its back in a vertically symmetrical orientation, orthogonally to the central plane separating the left and right side of the substantially symmetrical item. However, if the item lies tilted in a sorting channel it will be sorted according to a tilted width which is larger than the nominal width, and thus the item will not fall down through the sorting channel until later. If the item has an Irregular shape, such as a shrimp, its “tilted width” can be substantially larger than its nominal width, as illustrated in FIG. 4. This problem of non-regular orientation of the items decreases the accuracy and possible resolution of the grading and means that graded batches may contain a substantial portion of under-size items, i.e. items that are conveyed too far along the sorting channel and fall through the channel too late and into a grade intended for larger items.
When a prior art ridge grading machine was tested, with 5 m long ridge belts forming sorting channels that are 8-18 mm wide (a width increase of 2 mm/m) 20-70% of items (capelin or shrimp) were irregularly positioned and such items were conveyed up to 60 cm further down the sorting channels than if they were oriented properly. This means that a substantial portion of the items will be sorted in an incorrect size batch.